1. Field of the Invention
The present invention relates to an organic electro luminescence (EL) display panel device and a method of fabricating the same, and more particularly to an active matrix type organic electro luminescence display panel device and a method of fabricating the same having extended life span and improved light emission efficiency.
2. Discussion of the Related Art
In general, an organic EL display panel device is a passive matrix type or an active matrix type. A passive matrix type organic EL display panel device does not include thin film transistors (TFTs) separately. In the passive matrix type organic EL display panel device, gate lines cross data lines to form a matrix and the gate lines are sequentially driven to drive each pixel of the device. However, an instantaneous brightness is required to produce an average brightness across a number of lines to display images. Thus, if there are more lines in the device, higher voltage and current also required. Therefore, the passive matrix type organic EL display panel device has limited resolution, power dissipation and life span.
In contrast, an active matrix type organic EL display panel device includes a thin film transistor located at each pixel functioning as a switch for opening and shutting each pixel. A voltage applied to the pixel is charged to a storage capacitor and the charged voltage in the storage capacitor acts to apply power source until the next frame signal is applied. Thus, the active matrix type organic EL display panel device is driven for one frame period regardless of the number of gate lines. Therefore, the active matrix type organic EL display panel device has better resolution, power dissipation and life span in comparison to a passive matrix type organic EL display panel device.
FIG. 1 is a diagram of a pixel structure of an active matrix type organic EL display panel device according to a related art. In FIG. 1, the basic pixel structure of the active matrix type organic EL display panel device includes a gate line GL formed in a first direction; a power supply line VDD and a data line DL formed in parallel in a second direction crossing the first direction at a predetermined interval; and a pixel area defined by the crossing of the gate line GL, the data line DL and the power supply line VDD. In addition, the active matrix type organic EL display panel device includes a switching thin film transistor STFT connected to a storage capacitor Cst and the power supply line VDD, and an organic EL diode OLED connected to a drive thin film transistor MTFT. The switching TFT STFT controls the drive of the drive TFT MTFT in response to a selection signal from the gate line GL.
In the organic EL diode OLED, if an organic luminous material is supplied with a forward current, holes and electrons are moved to a light emission layer formed between a hole transport layer and an electron transport layer through a hole injection layer, the hole transport layer, the electron transport layer and an electron injection layer that are deposited between an anode electrode supplying holes and a cathode electrode supplying electrons. The moved holes and electrons are combined together within the light emission layer to generate a designated energy, which causes to emit light.
Further, if a signal is applied to a pertinent electrode in accordance with the selection signal, the switching TFT STFT is turned on. At this moment, a data signal is applied to the drive TFT MTFT and the storage capacitor Cst through the switching TFT STFT. If the drive TFT MTFT is on, a current from the power supply line VDD is applied to an organic EL layer through the drive TFT MTFT. In this case, the open and close time of the drive TFT MTFT becomes different in accordance with the size of the data signal, gray levels can be expressed by way of controlling the amount of current flowing through the drive TFT MTFT. Thus, the organic EL display panel can emit light continuously until the signal of the next frame is applied after a data charged in the storage capacitor Cst is continuously applied to the drive TFT MTFT.
According to the driving principle, the active matrix type organic EL display panel can apply the voltage lower and the current instantaneously lower than the passive matrix type organic EL display panel, and the organic EL display panel can be continuously driven for one frame period regardless of the number of selected lines. Thus, the active matrix type organic EL display panel is advantageous for low power dissipation, high resolution and a large screen. On the other hand, the active matrix type organic EL display panel has a structure where a current flows through a TFT, a polycrystalline silicon p-Si TFT is required which has a uniform crystalline state so as for the electric field effect mobility to be excellent because the related art amorphous silicon a-Si TFT is difficult to be adopted because silicon particles of non-crystalline state of the amorphous silicon causes electric field effect mobility to be low.
The polycrystalline silicon TFT has high electric field effect mobility, thus a drive circuit can be made on a substrate. Hereby, when the drive circuit is made on the substrate with the polycrystalline silicon TFT, the cost and mounting of the drive integrated circuit IC can be simplified. Polycrystalline silicon commonly is formed using a low temperature crystallization method including laser annealing of amorphous silicon.
FIG. 2 is a sectional view of an active matrix type organic EL display panel device according to the related art. In FIG. 2, the organic EL display pane device includes an insulating substrate 1, a buffer layer 30 formed on an entire surface of the substrate 1, a thin film transistor T formed in a first region of the buffer layer 30, a storage capacitor Cst formed in a second region of the buffer layer 30, and an organic EL diode E formed in a light emission region I on the substrate 1. The thin film transistor T includes an active layer 32 formed on the buffer layer 30, a gate electrode 38 formed on the active layer 32, and source and drain electrodes 50 and 52 on the active layer 32. The storage capacitor Cst includes a capacitor electrode 34 formed on the buffer layer 30 and a power electrode 42 formed opposite to the capacitor electrode 34 with a first insulating layer 40 therebetween. In addition, the organic EL diode E includes an anode 58 formed on a third insulating layer 54, and a cathode 66 formed opposite to the anode with an organic EL layer 64 therebetween.
In addition, the source electrode 50 of the thin film transistor T extends over a second insulating layer 44 and contacts the power electrode 42 of the storage capacitor Cst. Also, the anode 58 of the organic EL diode E extends over the third insulating layer 54 and contacts the drain electrode of the thin film transistor T.
FIGS. 3A-3I are sectional views of a method of fabricating the active matrix type organic EL display panel device of FIG. 2. In FIG. 3A, the buffer layer 30 is first formed on an entire surface of the insulating substrate 1. The buffer layer 30 is formed by depositing a first insulating material on the substrate 1. Then, the active layer 32a and the capacitor electrode 34 are formed on the buffer layer 30. The active layer 32a and the capacitor electrode 34 are formed by depositing polycrystalline silicon on the buffer layer 30 and patterned by a first mask process.
In FIG. 3B, a gate insulating film 36 and the gate electrode 38 are formed at a central area of the active layer 32a. The gate insulating film 36 and the gate electrode 38 are formed by depositing a second insulating material and patterned by a second mask process.
In FIG. 3C, the first insulating layer 40 is formed on the entire surface of the substrate 1 by depositing a third insulating material. Then, the power electrode 42 is formed on the first insulating layer 40 above the capacitor electrode 34.
In FIG. 3D, the second insulating layer 44 is formed on the entire surface of the substrate 1 by depositing a fourth insulating material and patterned. The second insulating layer 44 is patterned to form first and second ohmic contact holes 46a and 46b exposing regions 32b of the active layer 32a. The second insulating layer 44 is also patterned to expose a portion 48 of the power electrode 42. Then, the substrate 1 undergoes an ion doping process, thereby forming source and drain areas 1a and 1b containing impurities.
In FIG. 3E, the source and drain electrodes 50 and 52 are formed in the first and second ohmic contact holes 46a and 46b (shown in FIG. 3D) by depositing a third metal material and patterned by a fifth mask process. The source electrode 50 extends over the second insulating layer 44 and contacts the exposed region 48 (shown in FIG. 3D) of the power electrode 42.
In FIG. 3F, the third insulating layer 54 is formed on the substrate by depositing a fourth insulating material and patterned by a sixth mask process. The third insulating layer 54 is patterned to form a drain contact hole 56 exposing a portion of the drain electrode 52.
In FIG. 3G, the anode 58 is formed on the substrate 1 by depositing a transparent conductive material and patterned by a seventh mask process. The anode 58 is on the third insulating layer 54 and contacts the drain electrode 52 through the drain contact hole 56 (shown in FIG. 3F).
In FIG. 3H, a protective layer 60 is formed on the substrate 1 by depositing a fifth insulating material and patterned by an eighty mask process. The protective layer exposes a portion 62 of the anode 58. The protective layer 60 covers the thin film transistor T and protects the thin film transistor T from moisture and impurities.
In FIG. 3I, the organic EL layer 64 and the cathode 66 is formed on the substrate 1. In particular, the organic EL layer 64 contacts the exposed portion 62 (shown in FIG. 3H) of the anode 58, and the cathode 66 is formed on the entire surface of the substrate 1.
Accordingly, the organic EL display panel has a lower light emission scheme where a light emitted in the organic EL layer 64 comes out toward the substrate 1. Accordingly, its light transmittance is deteriorated because light is transmitted through the first to third insulating layers 40, 44 and 54 and the buffer layer 30. In addition, the organic EL display panel has a reduced light emission efficiency. For example, Formula 1 calculates the light emission efficiency of the organic EL display panel based on optics principles.ηextl=1/(2n2)×ηintl=1/(2×1.52)=⅕˜20%   [Formula 1]‘η’ represents internal or external light emission efficiency, and ‘n’ represents the refractive rate of a pertinent substrate. The refractive rate n of a substrate on which a buffer layer and an insulating layer are deposited in the related art is 1.5. Thus, in the organic EL display panel according to the related art, there is a disadvantage in that only 20% of the light emitted from the organic EL layer 64 to be transmitted through the substrate 1 is utilized on the display panel.